1. Field of the Invention
This invention relates to electromagnetic radiation diffuisers and more specifically, this invention relates to tuners used in reverberation chambers and their mounting in reverberation chambers.
2. Background of the Invention
Modern environments are awash in electromagnetic radiation due to the concentration, of transmitting devices. Also, most electrical equipment generates electromagnetic radiation during operation. Some electrical devices are adversely effected by this ubiquitous radiation. For example, devices associated with anti-lock braking systems in automobiles should be immune from rf frequencies emanating from electric motors or two-way radios operating nearby. Therefore, specific:testing standards mandate that some devices be tested in a controlled environment.
Reverberation chambers, anechoic chambers, and open area test sites (OATS) provide these controlled environs. However, reverberation chambers are more advantageous, inasmuch as test E-fields in the hundreds or thousands of volts per meter can be achieved using relatively low power sources such as 10 to 100 watt amplifiers.
Reverberation chambers are enclosures which contain electromagnetic radiation originating from within but exclude external ambient radiation. These chambers have inside surfaces which reflect the impinging radiation. They are used primarily to test the immunity of electronic equipment toward certain frequencies. Such chambers may be used to determine the frequencies emanating from a xe2x80x9cdevice under testxe2x80x9d (DUT). Typical DUTs include computer equipment, magnetic resonance imagers, avionics equipment, automobiles, and cellular phones.
The use of reverberation chambers continues to increase due to manufacturer test standards mandating their use. Also, the data-gathering speed associated with reverberation chambers versus the other methods described supra make chamber testing the preferred mode of analysis. As such, DUT traffic through reverberation chambers is increasing. To handle this increased traffic, reverberation chamber operations should assure rapid and accurate data collection.
Electromagnetic Comparability (EMC) testing standards which mandate or permit use of reverberant chambers require chamber calibration factors and chamber uniformity data. While this information is typically collected when the chamber is installed, recalibration and therefore retesting is required on a periodic basis. To avoid chamber down-time, this recalibration or retesting must be performed on an expedited basis.
Conventional methods of chamber testing include large metallic paddles or stirrers to mix, tune, or stir the fields within the chamber. This ensures that DUTs are exposed to uniform field strengths. The frequencies at which eigenmodes exist in a rectangular cavity of dimensions a, b and d are:       f    mnl    =            c              2        ⁢        Pi        ⁢                                            u              r                        ⁢                          xe2x80x83                        ⁢                          ϵ              r                                            ⁢          xe2x80x83        ⁢                                        (                                          (                mPi                )                            /              a                        )                    2                +                              (                                          (                nPi                )                            /              b                        )                    2                +                              (                                          (                lPi                )                            /              d                        )                    2                    
where c=speed of light;
m, n, and l are integers;
ur=relative magnetic permeability; and
∈r=relative dielectric permeability.
During recalibration, and to assure field homogeneity in the enclosures, particularly at frequency ranges of between 70 MHz and 150 MHz, tuners or frequency mixers are used to maximize boundary conditions or conversely, minimize the development of multiples of the frequencies studied. The tuners must be electrically large with respect to the enclosure and for the intended lowest frequency. Field homogeneity is achieved by exploiting the pseudo-statistical nature of each of the eigenfunction""s contribution to the field level at a given point within the chamber volume.
Simple reflective surfaces in the shape and having the operation of paddles or fans often are utilized as tuners. Other configurations include Z-fold surfaces which resemble stair steps arranged either along the horizontal, vertical or intermediate axes of the enclosure. Such Z-fold surfaces are rotated about the longitudinal axis of the Z-fold assembly.
Tuners are utilized either as mode-stirrers or mode tuners. Mode stirrers provide continuous movement of the reflective surfaces of the tuners. The data is measured for many different rotational positions of the tuner and for each frequency at any given power level. This is because field uniformity inside the volume of the chamber can only be obtained by averaging the fields influenced by the tuner over each tuner rotation. For optimum performance, these stirrers must exhibit low-flutter during their operation. Typical equilibration problems associated with mechanical stirrers have prompted some researchers to develop electronic mode stirring devices (See U.S. Pat. No. 5,327,091 issued to Loughry on Jul. 5, 1994). However, such electronic devices are relatively costly compared to their mechanical counterparts.
A mode tuner, on the other hand, is xe2x80x9csteppedxe2x80x9d through its complete rotation, with frequency measurements taken after the tuner stops at each step. To expedite testing using mode tuners, minimal vibration or settling time is desired. It is also possible to set a frequency and step the tuner through its required positions.
Mode tuners aregenerally more desirable when testing at lower frequencies. A typical test sequence will include turning a tuner to a given position, stepping (or sweeping) the frequency source through the desired range of frequencies, and then incrementing the tuner to the next position for a cycle repeat. However, if tuners with fast settling times are available, the tuner can be stepped through all if its required positions while measurements are taken at a single frequency.
The operation of mode tuners requires fast, precision mechanical tuner drivers and a tuner structure that either requires little mechanical dampening or which has naturally fast settling times. However, it is common for some fan tuners to exhibit settling times of up to 10 seconds, which is unacceptable in high through-put scenarios. As immunity test standards at lower frequencies increase, the larger tuners required to accommodate the longer wavelengths of these frequencies will tend to have larger rotational moments due to their increased mass. These larger tuners represent a challenge for maintaining tight, fast control of their stepped positions. While precision stepper motors provide the needed drive, total mass of the tuner structure is critical, as is optimizing feedback settings of the motor controller amplifiers.
Many reverberation chamber tuners have a single support structure or center shaft from which electrically conductive surfaces are radially mounted. ( See, for example, U.S. Pat. No. 5,530,412 issued to Goldblum on Jun. 25, 1996.) The larger the surfaces, the higher the rotational moment of inertia associated with their movement. This leads to increased settling times, and therefore the problems discussed, supra
A need exists in the art for tuners or stirrers which contain large conductive surfaces. The tuners/stirrers should also exhibit minimal settling times, while also maintaining relatively low mass. The mounting and mechanical drive means for the tuners or stirrers also should minimize the introduction of electromagnetic radiation into the chamber in which they are operating.
It is an object of the present invention to provide a device for tuning reverberation chambers that overcomes many of the disadvantages of the prior art.
Another object of the present invention is to provide a tuner for a reverberation chamber to accommodate electromagnetic radiation frequencies of as low as 20 MHz. A feature of the invention is a plurality of support members juxtaposed to electrically conductive surfaces of the tuner. An advantage of the invention is the short settling time (i.e. fast dampening time) displayed by the supported tuner.
Yet another object of the present invention is providing a tuner which minimizes the encroachment of electromagnetic radiation into a reverberation chamber. A feature of the invention is a bearing feed-through mechanism for mounting the actuating shaft of the tuner that eliminates rf leakage from the outside to the inside of the enclosure and just as importantly, from the inside of the enclosure to the outside. Another feature of the invention is the use of an externally situated actuating motor for moving the tuner. An advantage of the invention is that the tuner does not add to the burden of excluding ambient electromagnetic radiation from the chamber. Another advantage is that the need for additional filters and the compensation for metallic motor structures is obviated.
Briefly, the invention provides for a tuner in a reverberation chamber comprising a plurality of electrically conductive surfaces attached to each other in a Z-fold configuration so as to form an elongated substrate having a longitudinal axis, two lateral edges extending generally parallel to the longitudinal axis, and two leading edges extending generally perpendicular to the longitudinal axis; a first and second planar substrate attached to each lateral edge; and a third and forth planar substrate attached to each leading edge and also attached to each first and second planar substrate.
Also provided is a device for minimizing electromagnetic radiation leakage along a shaft comprising a bearing housing; a plurality of thrust bearings contacting the bearing housing and axially juxtaposed along the shaft so that at least two thrust bearings oppose each other; a radial bearing intermediate each thrust bearing; and a means for moving the thrust bearings toward each other so as to compress the thrust bearings against the bearing housing.